It is known, for instance from U.S. Pat. No. 3,649,979 to MacNiel; U.S. Pat. No. 3,913,158 to Vilarrubis; U.S. Pat. No. 4,083,071 to Forjot and French Patent Application No. 2,355,529 to Beuchat to provide swim fins with a passage extending longitudinally through the fin from an inlet remote from the free end of the fin to a discharge at the free end of the fin, so that in the ordinary use of the fin water will be drawn into the inlet and discharged from the free end to impart additional thrust to the swimmer.
The said prior art known fins are effective in their intended action only during one stroke of the fin. Moreover, due to their construction, they are unduly rigid and heavy and therefore overstraining.
It is further known from U.S. Pat. No. 3,411,165 to Murdoch a swim fin comprising: a semirigid shoe-like member from which inflexible ribs are extending forwardly at each side thereof; and a thin web-like membrane wider than the space between said ribs, secured to the sides of said ribs and to the front of the shoe-like member so that said web-like membrane may belly oppositely between said ribs during swimming.
The said prior art fin to Murdoch has many disadvantages. First of all, it may be said that it is very difficult to make the ribs of such a fin substantially inflexible in vertical planes without providing them with an internal metallic stiffening member. This implies that the shoe-like member supporting the said stiffening members of the ribs be made form a semirigid material in order to resist to the forces tending to flex said ribs in vertical planes. This is a remarkable disadvantage for a fin, since it is known that the overall tendency is to make the shoe-like member of a fin with a material as supple as possible, in order to afford greater comfort to the user's feet, thus avoiding injuries to the feet. Moreover, the said ribs are subjected to forces tending to flex them in horizontal planes, thus reducing the projection of the active surface of the web-like membrane during each swimming stroke, which reduces the propulsive efficiency of the fin at each swimming stroke.
From French Patent Application No. 2,506,619 to Beuchat et al., a swim fin is known comprising a shoe portion connected to a first rigid portion of the fin blade, from which two lateral reinforcing ribs extend forward in a diverging manner. Between the said ribs a supple flexible middle portion is disposed, connected to the reinforcing ribs by two sections in the shape of half truncated cones, which forms the blade of the fin. The said known fin presents the same disadvantages mentioned with respect to the Murdoch fin (U.S. Pat. No. 3,411,165) in respect of the reduction of its active surface during each swimming stroke. Moreover, it is known that, the more the fin blade is supple, the more it is subject to elastic deformation, with the result that the thrust on the swimmer is partially lost. This is particularly true at the points of inversion of the stroke of the fin, at which same is caused to flex from one position to the new one, with consequent additional loss of propulsive power. This is the main reason because there is the tendency to make the blades of the fins as rigid as possible in order to achieve the best results.
In the fin according to the above discussed French Patent Application No. 2,506,619 the above mentioned advantages are not achieved, or are achieved only partially, due to the fact that the main portion of the fin blade is made of supple material.